Oncogene studies on spontaneous and chemically induced rodent neoplasms, started 5 years ago in collaboration with molecular biologists in the Division of Biometry and Risk Assessment, are continuing. Emphasis has been on the patterns of oncogene activation in chemically induced versus spontaneous lung and liver neoplasms using samples from NTP bioassays as well as from rodent studies designed to investigate strain susceptibility to genotoxic and nongenotoxic chemical carcinogens. Important information useful for risk assessment and understanding mechanisms of carcinogenesis has been generated from past studies and underscores the need to continue these studies over the next two to three years.